The invention relates to a metering pump assembly for admixing a fluid reduction agent in an exhaust gas flow.
With auto-igniting internal combustion engines, nitrous oxides arise on operation with excess oxygen, which is the case in the majority of operating conditions, and specifically with direct injection into the main combustion space, as is typically the case with diesel motors. It is known to supply a reduction catalyser to the exhaust gas flow, in order to reduce these nitrous oxide emissions. Usually, an aqueous urea solution as a reduction agent is supplied to the exhaust gas in a finely distributed manner before entry into the catalyser. Thereby, the supplied urea quantity is to be matched as exactly as possible to the combustion process, in order to ensure an as complete as possible reduction within the catalyser, and on the other hand to avoid an excess of urea.
A special metering pump for metering and supplying the reduction agent to the exhaust gas flow is known from EP 1 435 458 A1. With this metering pump, the supplied reduction agent is mixed or subjected to pressurized air in the metering pump assembly before the supply to the exhaust gas flow. Because urea tends to crystallize in combination with air, it is important to flush the complete region which comes into contact with the air and urea, with air, before the stoppage of the installation, so that no urea remains in this region. For this, a 3/2-way valve is provided in the metering pump according to EP 1 435 458 A1, which in a first position leads urea into a conduit section which is connected to a pressurized air conduit for impinging the urea with pressurized air. In a second position, the 3/2-way valve is switched such that no urea gets into the mentioned conduit section, but pressurized air is supplied via a second entry of the 3/2-way valve, so that the conduit section in which the urea is subjected to pressurized air, is flushed out with pressurized air. In this manner, one may prevent a crystallization of the urea in the conduit section.
The construction of the metering pump assembly according to EP 1 435 458 A1 is quite complicated on account of the valve required for the flushing. Furthermore, the operation of the metering pump assembly is made more complicated, since the flushing of the installation part coming into contact with air and urea must be ensured every time when switching off. However, for reasons of cost and for reasons of operational reliability, it is desirable to simplify the construction and operation of the metering pump assembly.